The present invention refers to a fully differential, switched capacitor, operational amplifier circuit with common-mode controlled output.
As is known, in the manufacture of numerous switched-capacitor circuits advantageously switched operational amplifiers with fully differential topology are used, i.e., having two output terminals supplying differential voltages that are symmetrical with respect to a common-mode voltage. Switched operational amplifiers, in fact, overcome certain drawbacks typical of traditional switched-capacitor circuits, especially if they operate at a low supply voltage (for example, just over 1 V) and with a low consumption. These drawbacks, in particular, are due to the difficulty, in the presence of low supply voltages, of efficiently driving switches connected to terminals having wide dynamics that are comparable to the rail-to-rail supply voltage, such as in case of the output of an operational amplifier. The fully differential topology, moreover, allows the manufacture of integrated circuits having small overall dimensions, for implementing, for instance, high order active filters.
In order to prevent distortion of the signals, however, it is necessary to associate fully differential switched operational amplifiers to circuits that perform an accurate control of the common-mode voltage. A control circuit of such a type is described in U.S. Pat. No. 5,973,537 issued on Oct. 26, 1999, according to which the outputs of a fully differential switched operational amplifier are connected, via respective switched capacitors, to the inverting input of a standard operational amplifier, in an integration configuration. Thereby, the output of the standard operational amplifier supplies a voltage that is proportional to the common-mode voltage and that can be used for modifying the biasing of the fully differential switched operational amplifier and for obtaining a preset common-mode voltage. In addition, the inverting input of the standard operational amplifier is alternately connected to ground and to the supply voltage via a further switched capacitor in order to prevent voltage jumps when the switched operational amplifier is turned on.
According to a different known solution, the outputs of the fully differential switched operational amplifier are connected to the supply line via a capacitive divider. An intermediate node of the capacitive divider, supplying a voltage correlated to the common-mode voltage and with a reduced dynamics with respect to the differential voltages of the switched operational amplifier, is connected to the inverting input of a standard operational amplifier, which also receives, on its non-inverting input, a reference voltage. The output voltage of the standard operational amplifier, correlated to the difference between the common-mode voltage and the reference voltage, is used for modifying the biasing of the switched operational amplifier and for obtaining a preset common-mode voltage, as in the previous case.
The above solutions, however, have a number of drawbacks. In both cases, in fact, an additional standard operational amplifier must be used, which increases circuit consumption and, furthermore, limits the performance of the circuit in terms of frequency response. The increase in power absorbed due to the standard operational amplifier is particularly disadvantageous when the circuit is used inside a very low consumption device. The second solution, moreover, requires the use of supply voltages higher than that of the switched operational amplifier.
The disclosed embodiments of the present invention provide a control circuit that overcomes the above described drawbacks and, in particular, results in a very low consumption, uses the same supply voltage as for the switched operational amplifier, and reduce the number of components.
In accordance with one embodiment of the present invention, a fully differential, switched capacitor, operational amplifier circuit with common-mode controlled output is provided, the operational amplifier circuit including a control circuit having a capacitive detecting network and a biasing control circuit. The capacitive detecting network includes a first capacitive element connected between a first differential output of the operational amplifier and a common-mode node, a second capacitive element coupled between the second differential output of the operational amplifier and the common-mode node, and a third capacitive element connected between the common-mode node and a biasing node that is connected to a first reference potential line in a first operative condition and a second reference potential line in a second operative condition. The biasing control circuit includes a control input connected to the common-mode node and an output connected to a control terminal of the operational amplifier. A switchable biasing circuit is further included that is activated for biasing the common-mode node at a desired voltage in a first operative condition and is deactivated in a second operative condition to leave the common-mode node in a floating condition.